This invention relates to mechanical gear boxes and more particularly to a two speed gear box with a hydraulic piston actuated shifter and an isolator to isolate the gears from the force of the hydraulic shifting piston.
Many mechanical drive arrangements require a gear box in the drive train to change the speed and the torque of the driver to a speed and torque suitable for the load. Often the input to the gear box is from a high speed electric or hydraulic motor and the output is a low speed high torque drive to the input shaft of the load. A particularly apt gearing arrangement to achieve a high ratio gear change is the planetary gear box.
In many applications it would be very desirable to have the capability of shifting between two gear ratios in the gear box. One application for such a device would be in the drive train of large construction or mining equipment. These large vehicles are often driven independently at each wheel by hydraulic motors acting through reduction gear boxes. In its periods of normal operation such as ascending a steep grade with a heavy load or working the earth, the gear box would be in its high reduction configuration, for example, 3.47:1, and the speed of the vehicle would be determined by the output of a variable capacity hydraulic pump connected to the vehicle prime mover. Then, when the vehicle is to be moved on level terrain without load or when it is being transferred between work stations, a much higher operating speed would be desired to effect the rapid transfer. To accomplish this, the gear box would be shifted from its high reduction mode to its directly coupled 1:1 drive mode so that the speed range within the operating range of the hydraulic pump is much higher.
There have been a few attempts to produce a mechanically reliable two speed planetary gear box without clutches which permits a shift between a 1:1 drive ratio and a high reduction drive ratio. However, these devices all suffer from the same basic defect, namely, the hydraulic pressure needed to reliably shift the speed reducer produces a sudden and occasionally nonmeshing engagement between the gears when they shift which can damage or destroy the gears. Also, the speed limitations of hydraulic shifters make partial engagement of the shifter possible, which can lead to damage to the gears and the shifter. In addition, it may be desirable to provide a shifting force which is high enough to disengage the gear box even under load, but should provide only a gentle engaging pressure so that the gears will not be forced into engagement under load and be damaged in the process.